STOMACH

The Esophagus: Anatomy and Physiology

• Structural Composition of the Esophagus:

  • The esophagus is a muscular tube that facilitates the passage of food from the pharynx to the stomach.

  • Muscle Types: The top $1/3$ of the tube is composed of skeletal muscle, while the lower $2/3$ is composed of smooth muscle.

  • Epithelium: It is lined with stratified squamous epithelium.

    • "Stratified" refers to the arrangement in multiple layers.

    • "Squamous" refers to the flattened shape of the cells.

    • Function: This specific cell layering protects the underlying regions of the esophagus from the physical abrasion caused by food as it passes through.

  • Absorption: There is no absorption of nutrients in the esophagus.

  • Secretion: Mucus is secreted throughout the esophagus to provide lubrication and aid in the smooth passage of the food bolus.

• Esophageal Sphincters:

  • Upper Esophageal Sphincter (UES): Located just below the pharynx; it consists of a ring of skeletal muscle.

  • Lower Esophageal Sphincter (LES): Located at the junction where the esophagus meets the stomach; it consists of a ring of smooth muscle.

  • State of Sphincters: Both sphincters remain closed under normal conditions, except during specific actions: swallowing, vomiting, or burping.

The Esophageal Phase of Swallowing and Heartburn

• The Process of Swallowing:

  • The esophageal phase begins when the upper esophageal sphincter relaxes, allowing the food bolus to enter the esophagus.

  • Once the food has passed into the tube, the upper sphincter closes and the glottis opens, allowing the individual to resume breathing.

  • Peristaltic waves (rhythmic muscular contractions) move the food bolus down the esophagus toward the stomach.

  • This transit time typically lasts between $5$ and $9$ seconds.

  • As the bolus reaches the end of the esophagus, the lower esophageal sphincter opens to allow food to enter the stomach, promptly closing afterward.

  • Propulsion Force: The main force facilitating this phase is peristalsis; gravity is not necessary for swallowing to occur.

• Heartburn and Acid Reflux:

  • The primary function of the lower esophageal sphincter (LES) is to prevent acidic gastric contents from flowing back into the esophagus.

  • This prevention is aided by an equal pressure environment between the lower esophagus and the stomach, meaning there is no pressure gradient to force contents upward.

  • Self-Correction Mechanisms: When small amounts of acid do enter the esophagus, two mechanisms are triggered to clear it:

    1. Stimulation of peristalsis to move the acid back into the stomach.

    2. Increased salivary secretion; the saliva is alkaline and helps neutralize the acid while aiding in clearance.

  • Causes of Heartburn: Heartburn occurs when the LES does not close properly. Common triggers include:

    • Consumption of a very large meal.

    • Pregnancy (due to increased abdominal pressure and hormonal changes).

The Stomach: Overview and Anatomy

• Definition and General Function:

  • The stomach is a muscular, sac-like organ located between the esophagus and the small intestine (SI).

  • It serves three primary functions:

    1. Storage: It holds food before it is processed by the small intestine.

    2. Mechanical Breakdown: It physically grinds food into smaller pieces.

    3. Chemical Breakdown: It initiates the digestion of macromolecules through specific secretions.

• Key Gastric Secretions:

  • Pepsinogen: An inactive precursor (zymogen) that is cleaved to form the enzyme pepsin. Pepsin is critical for initiating protein digestion.

  • Hydrochloric Acid ($HCl$): This acid dissolves food and partially digests macromolecules. It also serves a protective function by providing "partial" sterilization of the food.

  • Chyme: The result of the stomach's mechanical and chemical processes—including protein fragments, polysaccharides, fat droplets, salt, and water—is known as chyme.

  • Intrinsic Factor: A glycoprotein required for the absorption of Vitamin $B_{12}$ in the ileum.

    • Vitamin $B_{12}$ is essential for normal red blood cell (RBC) formation.

    • Failure to absorb Vitamin $B_{12}$ results in a condition called pernicious anemia (RBC deficiency).

• Absorption in the Stomach:

  • The stomach is not a significant absorption site.

  • A very small amount of water and alcohol are the primary substances that can be absorbed across the gastric mucosa.

• Anatomical Regions:

  • Fundus and Body: These comprise the upper portion of the stomach. They have a thin layer of smooth muscle and are the sites where mucus, pepsinogen, and $HCl$ are secreted.

  • Antrum: The lower region of the stomach. It features a much thicker layer of smooth muscle for mixing and grinding food. It secretes mucus, pepsinogen, and the hormone gastrin.

  • Pyloric Sphincter: Located at the exit of the antrum; it controls the rate at which chyme empties into the small intestine.

Gastric Secretions: Cell Types and Glands

• Exocrine vs. Endocrine Secretions:

  • Exocrine: Messengers secreted into ducts and onto epithelial surfaces (e.g., mucus, $HCl$, pepsinogen).

  • Endocrine: Messengers secreted into the blood to reach distant targets (e.g., gastrin).

  • Paracrine: Messengers that act locally on neighboring cells (e.g., histamine, somatostatin).

• The Generalized Gastric Gland:

  • Mucous Cells: Located at the luminal (top) end of the gland; they produce mucus to protect the stomach lining from self-digestion.

  • Parietal Cells (Oxyntic Cells): Found mostly in the body and fundus; they secrete $HCl$ and intrinsic factor.

  • Chief Cells: Found in gastric glands in all regions; they secrete pepsinogen.

    • Pepsinogen is a zymogen, meaning it is inactive until a chemical reaction (cleavage by stomach acid) turns it into pepsin.

  • Enteroendocrine Cells ($G$ Cells): Primarily found in the antrum; they secrete the hormone gastrin into the blood to stimulate $HCl$ production and GI motility.

  • Enterochromaffin-like Cells (ECL Cells): Found in all regions (though more prevalent in the antrum); they secrete the paracrine histamine, which stimulates $HCl$ production.

  • D Cells: Found in all regions (more in the antrum); they secrete the paracrine somatostatin, which inhibits $HCl$ production.

Mechanism of Parietal Cell Acidification

• Cellular Morphology of Parietal Cells:

  • The surface of the parietal cell is modified with canaliculi (small channels).

  • In an inactive state, canaliculi are small. When activated, membranes move to these channels, distending them to increase surface area for maximum acid secretion.

  • Parietal cells contain numerous mitochondria to generate the ATP required for active transport of acid.

• Transporters and Ion Movement:

  1. $Na^+/K^+$ ATPase: Located on the basolateral side; it pumps $3\text{ Na}^+$ out and $2\text{ K}^+$ in per ATP hydrolyzed, establishing electrochemical gradients.

  2. $H^+/K^+$ ATPase (The Proton Pump): Located on the apical (luminal) membrane; it uses primary active transport to pump protons ($H^+$) into the stomach lumen in exchange for $K^+$ ions.

  3. Carbonic Anhydrase: To keep the cell from becoming too basic as $H^+$ leaves, this enzyme catalyzes: $H_2O + CO_2 \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$. The $H^+$ is used for the proton pump.

  4. $Cl^-/HCO_3^-$ Exchanger: On the basolateral membrane, the base bicarbonate ($HCO_3^-$) is pumped out in exchange for chloride ($Cl^-$) through secondary active transport.

  5. $K^+$ Channels: Open on the apical surface to allow $K^+$ to recycle back into the lumen down its concentration gradient.

  6. $Cl^-$ Channels: Open on the apical membrane to allow negative chloride ions to follow the positive charge loss into the lumen.

  • Result: $HCl$ is formed in the lumen from the independently secreted $H^+$ and $Cl^-$ ions.

Regulation of Gastric Acid Secretion

• Key Regulatory Messengers:

  • Gastrin (Hormone): Released by $G$ cells; stimulates the insertion of $H^+/K^+$ ATPases into the membrane.

  • Acetylcholine (ACh - Neurotransmitter): Released by parasympathetic nerves; increases $H^+/K^+$ ATPase insertion and stimulates acid production.

  • Histamine (Paracrine): Released by ECL cells; stimulates the insertion of the proton pump. It notably potentiates the effects of gastrin and ACh.

  • Somatostatin (Paracrine): Released by $D$ cells; inhibits the secretion of $HCl$, gastrin, and histamine.

• The Three Phases of Gastric Secretion:

  1. Cephalic Phase: Initiated by the brain (sight, smell, or taste of food). Excitatory signals via the vagus nerve release ACh at the parietal cell.

  2. Gastric Phase: Triggered when food reaches the stomach. This is the major stimulatory phase. Stimulated by the presence of food causing $G$ cells to release gastrin.

  3. Intestinal Phase: Occurs when chyme enters the duodenum. This is the inhibitory phase. Presence of acid, fat, and hypertonic solutions triggers the release of hormones like secretin and cholecystokinin (CCK), which inhibit gastrin and acid production.

• Feedback Loops:

  • ACh not only stimulates parietal cells directly but also stimulates ECL cells (to release histamine) and $G$ cells (to release gastrin) while inhibiting $D$ cells (to prevent somatostatin).

  • Once acid levels are high, $H^+$ ions directly inhibit $G$ cells to reduce gastrin release.

  • As ACh levels drop, the inhibition on $D$ cells is lifted; somatostatin then acts to inhibit parietal cells, ECL cells, and $G$ cells.

Gastric Motility and Electrical Activity

• Mechanical Actions:

  • Receptive Relaxation: As a meal is consumed, smooth muscle relaxes via parasympathetic/enteric mediation, allowing the stomach to stretch with minimal pressure increase.

  • Peristaltic Waves: Weak contractions begin in the body; the antrum then contracts powerfully to grind and mix food.

  • Retropulsion: During antral contraction, the pyloric sphincter closes. Only a tiny amount of chyme enters the duodenum; the rest is forced backward into the stomach body for further mixing.

• Electrical Basis:

  • Pacemaker Cells: Located in the smooth muscle layer; they create spontaneous slow waves of depolarization and repolarization.

  • Basic Electrical Rhythm (BER): These waves determine the frequency of contractions. Without neural/hormonal input, these waves are too small to cause actual contraction.

  • Contractile Strength: Excitatory hormones and neurotransmitters further depolarize the membrane. The level of stimulus determines the strength of the contraction.

Gastrointestinal Complications: Vomiting and Ulcers

• Vomiting (Emesis):

  • Causes: Psychogenic factors, GI disturbances, inner ear infections, toxins detected by chemoreceptors, or CNS pressure.

  • The Process: Stimuli feed into the vomiting center in the medulla oblongata. Salivation and nausea occur; the glottis closes; the abdominal muscles and diaphragm contract; the LES relaxes.

  • Reverse Peristalsis: Moves intestinal contents back into the stomach, followed by the movement of gastric contents up the esophagus and out the mouth.

  • Consequences:

    • Benefits: Removal of toxins and negative conditioning against noxious substances.

    • Negatives: Dehydration, electrolyte imbalance, metabolic alkalosis (elevated pH due to loss of $HCl$), and erosion of tooth enamel.

• Peptic Ulcers:

  • Definition: Erosion of the GI mucosa in acidic regions (esophagus, stomach, duodenum).

  • Cause: An imbalance between aggressive factors ($HCl$, pepsin) and protective factors (mucus, bicarbonate).

    • The most common cause is the bacterium Helicobacter pylori.

    • Non-bacterial causes include NSAIDs (which reduce prostaglandins), smoking, alcohol, and gastrinomas.

  • Treatment: Antibiotics for $H. pylori$, proton pump ($H^+/K^+$) inhibitors, histamine receptor antagonists ($H_2$ blockers), and prostaglandin-type drugs.

Gastric Bypass and the Pancreas

• Is the Stomach Essential?

  • Not strictly essential for life, but removal leads to significant issues.

  • Complications of Stomach Removal:

    • Loss of intrinsic factor necessitates $B_{12}$ injections to prevent anemia.

    • Loss of sterilization ($HCl$) increases infection risk.

    • Loss of regulation regarding food entry into the small intestine.

• Introduction to the Pancreas:

  • An organ with both endocrine and exocrine functions.

  • Exocrine Pancreas: Produces the majority of enzymes required to digest carbohydrates, proteins, fats, and nucleic acids, which are secreted into the GIT.